refactor: add pure attribute to pure functions (#18165)

This will allow the compilers that support the pure attribute to make
further optimizations.

src/nvim/buffer.c

@@ -357,6 +357,7 @@ void set_bufref(bufref_T *bufref, buf_T *buf)
 ///
 /// @param bufref Buffer reference to check for.
 bool bufref_valid(bufref_T *bufref)
+  FUNC_ATTR_PURE
 {
   return bufref->br_buf_free_count == buf_free_count
     ? true
@@ -2100,6 +2101,7 @@ buf_T *buflist_findname(char_u *ffname)
 ///
 /// @return  buffer or NULL if not found
 static buf_T *buflist_findname_file_id(char_u *ffname, FileID *file_id, bool file_id_valid)
+  FUNC_ATTR_PURE
 {
   // Start at the last buffer, expect to find a match sooner.
   FOR_ALL_BUFFERS_BACKWARDS(buf) {
@@ -2531,7 +2533,7 @@ static bool wininfo_other_tab_diff(wininfo_T *wip)
 ///
 /// @return  NULL when there isn't any info.
 static wininfo_T *find_wininfo(buf_T *buf, bool need_options, bool skip_diff_buffer)
-  FUNC_ATTR_NONNULL_ALL
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_PURE
 {
   wininfo_T *wip;
 
@@ -2609,6 +2611,7 @@ void get_winopts(buf_T *buf)
 ///
 /// @return  a pointer to no_position if no position is found.
 pos_T *buflist_findfpos(buf_T *buf)
+  FUNC_ATTR_PURE
 {
   static pos_T no_position = { 1, 0, 0 };
 
@@ -2618,6 +2621,7 @@ pos_T *buflist_findfpos(buf_T *buf)
 
 /// Find the lnum for the buffer 'buf' for the current window.
 linenr_T buflist_findlnum(buf_T *buf)
+  FUNC_ATTR_PURE
 {
   return buflist_findfpos(buf)->lnum;
 }
@@ -4928,6 +4932,7 @@ void do_arg_all(int count, int forceit, int keep_tabs)
 
 /// @return  true if "buf" is a prompt buffer.
 bool bt_prompt(buf_T *buf)
+  FUNC_ATTR_PURE
 {
   return buf != NULL && buf->b_p_bt[0] == 'p';
 }
@@ -5339,6 +5344,7 @@ bool bt_dontwrite_msg(const buf_T *const buf)
 /// @return  true if the buffer should be hidden, according to 'hidden', ":hide"
 ///          and 'bufhidden'.
 bool buf_hide(const buf_T *const buf)
+  FUNC_ATTR_PURE
 {
   // 'bufhidden' overrules 'hidden' and ":hide", check it first
   switch (buf->b_p_bh[0]) {